In wireless communication systems, an information signal is communicated from a transmitter to a receiver via a channel comprising several independent paths. These paths are referred to as multipaths. Each multipath represents a distinct route an information signal may take in traveling between the transmitter and receiver. An information signal communicated via such a channel--a multipath channel--appears at a receiver as a plurality of multipath signals, one signal for each multipath.
The amplitudes and phases of signals received from a transmitter through different multipaths of a channel are generally independent of each other. Because of complex addition of multipath signals, the strength of received signals may vary between very small and moderately large values. The phenomenon of received signal strength variation due to complex addition of multipath signals is known as fading. In a fading environment, points of very low signal strength, or deep fades, are separated by approximately one-half of a signal wavelength from each other.
Wireless communication channels can be described by certain channel characteristics, such as amplitude attenuation and phase shifting. For example, the multipaths of a channel may provide different amplitude attenuations and phase shifts to an information signal communicated from a transmitter to a receiver. These different amplitude and phase characteristics may vary due to e.g., relative movement between transmitter and receiver, or changes in local geography of the transmitter or receiver due to movement. Because of the variation of channel characteristics, a receiver can experience a signal whose strength varies with time. This variation is the manifestation of the complex addition of multipath signals having time varying amplitudes and phases.
If the characteristics of a multipath channel vary slowly, a receiver experiencing a deep fade may observe a weak signal for a long period of time. Long fades are not uncommon in, e.g., indoor radio systems, where relative movement between receivers and transmitters is slow or nonexistent (often, one of these two is an immobile base station; the other is a mobile device carried by a person). Since the duration of a deep fade in an indoor radio system may be large in comparison to the duration of information symbols being communicated, long bursts of symbol errors may occur (due to the weakness of received signal strength for an extended period of time).
Space diversity is a classical technique for mitigating the detrimental effects of fading, such as error bursts. Space diversity is provided through the use of a plurality of antennas at a receiver. If the receiver antennas are separated by more than a couple of wavelengths, the multipath signals received by the individual receiver antennas are approximately independent of each other. When several antennas are used by a receiver, the probability that received signals will yield a deep fade at all antennas simultaneously is small. Thus, signals received by these antennas may be combined to reduce the effects of fading.
Space diversity, however, is not without its drawbacks. For example, space diversity requires the use of a plurality of widely spaced antennas. For small portable receivers this requirement is problematic. Also, space diversity increases the complexity of a receiver, thereby increasing its cost.
Time diversity is another technique which has been employed to mitigate the detrimental effects of fading. Time diversity may be achieved by transmitting a plurality of copies of an information signal during distinct time intervals. These transmission time intervals should be separated in time so that received signals are subjected to independent fades. Once the plurality of signal copies have been received by a receiver, the independent nature of their fades facilitates avoidance of the detrimental effects of fading.
Like space diversity, time diversity also has its drawbacks. Time diversity is predicated on the idea of identical signal transmission at different times. However, the time needed to receive a plurality of copies of an information signal presents a delay in the communication process which may be undesirable, if not intolerable.
Time diversity can also be effectively obtained when a channel code is used in conjunction with an interleaver/deinterleaver pair well known in the art. An interleaver receives a set of consecutive channel coded data symbols for transmission and rearranges them in, e.g., a pseudorandom fashion. Typically, the number of symbols in the set extend for a duration beyond that of a slow deep fade. Rearranged symbols are transmitted over the channel to a receiver having a single antenna. By virtue of transmission, consecutive symbols are subject to similar fading. However, these consecutively transmitted symbols are not in original order. A receiver equipped with a deinterleaver rearranges the symbols back to their original order. Due to the randomness of their transmission order, data symbols presented to the channel decoder by the deinterleaver have been subject to essentially independent fades. The independent symbol fading afforded by the interleaver/deinterleaver pair may be utilized to avoid fading's detrimental effects.
However, as with the first time diversity technique discussed above, a transmission delay is created by this approach. This delay is directly proportional to the size of the interleaver. Allowable transmission delay imposes limits on the size of the interleaver. However, an interleaver of a size beyond the imposed limit may be needed to deal effectively with fading.